Present-day commercial gasification technologies (coal gasification, oil gasification) are based on oxygen-blown gasification reactors. The oxygen required for gasification is provided by an air separation plant. The air separation process used can be divided into the following main steps:
air compression PA0 air cleaning with heat exchange PA0 cryogenic rectification PA0 compressing of the product flows to the pressure level required for the process. PA0 low efficiency PA0 scarce availability PA0 low economy PA0 no flexibility in operation.
Since the air separation plant operates under a pressure ratio of 6 bar, the air is compressed to 6 bar in a first partial process, while the oxygen and nitrogen product flows are produced at ambient pressure.
However, air separation plants employed in combined cycle power plants with coal or oil gasification are often designed for a higher pressure level. The gas turbine compressor is used in these cases also as the compressor for air separation, i.e. the required air is exhausted downstream of the gas turbine compressor. The air separation plant is operated at the pressure level of the gas turbine compressor, for example at 14 bar, the product gases oxygen and nitrogen are then produced at approximately 4.5 bar. The nitrogen is again compressed to the gas turbine combustion chamber pressure level, is heated in counterflow to the extraction air and is mixed with the gas from the coal or oil gasification (synthetic gas).
Because synthetic gas has high proportions of hydrogen and carbon monoxide, it must be saturated with water if it is intended to attain the required maximum NO.sub.x threshold values by means of state of the art combustion techniques.
However, when burning synthetic gas saturated with water, a problem occurs with standard gas turbines, namely that the low calorific value of the synthetic gas requires large fuel mass flows. This effect is further increased by the additional water mass flow from the combustion gas saturation. As a result, the combustion chamber pressure of standard gas turbines increases and the surge limit of the compressor is exceeded. The known solution of bleeding air from the gas turbine compressor and the integration of the gas turbine process with the air separation plant, i.e. the return of the nitrogen into the gas turbine combustion chamber, is a result thereof.
This state of the art has the following disadvantages: